cells - MCAT

credit to khan academy

Describe the function of cholesterol in animal cell membranes.

Acts as buffer for fluidity

At high temperatures, membrane tends to become more fluid; cholesterol helps maintain rigidity

At low temperatures, membrane tends to become more rigid; cholesterol helps maintain fluidity

Why is the assembly of a cell membrane from free phospholipids a spontaneous process?

Free phospholipids are surrounded by water in an orderly fashion

When phospholipids assemble into a bilayer, it disturbs the orderly arrangement of water molecules

Entropy of phospholipids decreases but entropy of water increases by a greater amount, which corresponds to thermodynamic favorability of the process

What is the function of saturated and unsaturated fatty acid tails in the phospholipid bilayer?

Saturated fatty acids allow for tight packing, increasing membrane rigidity/integrity and conferring heat resistance

Unsaturated fatty acids prevent tight packing, increasing membrane fluidity and conferring freeze resistance

Describe the 4 main types of transport across a cell membrane

Simple diffusion = movement of small nonpolar molecules along concentration gradients

• eg. O₂ from alveoli into capillaries

Facilitated diffusion = movement of large molecules, polar molecules and/or ions along concentration gradients using channels

• eg. Glucose Transporters allow glucose in blood to enter cells

Primary active transport = movement of ions against concentration gradient which directly uses ATP & ATPase

• eg. Na⁺ / K⁺ pump consumes 1 ATP to move 2 K⁺ in and 3 Na⁺ out, both against their concentration gradients

Secondary active transport = movement of molecule with ion using concentration gradient set up by primary active transport (to allow passive diffusion of partner ion)

• eg. Sodium-glucose cotransporter (SGLT) in small intestine allows glucose reabsorption by cells which already have high concentration of glucose

3 types of endocytosis

• give example for each

Phagocytosis for undissolved molecules

• eg. Macrophage enveloping bacterium to break it down

Pinocytosis for dissolved solutes

• eg. microvilli in small intestine invaginate their membranes and ‘drink’ fluid to absorb dissolved nutrients

• extra eg. macrophages sampling environment for antigens/invaders

Receptor-mediated endocytosis for engulfing ligand + receptor

• eg. cellular intake of Low-Density Lipoprotein: LDL binds to receptors on surface, then surface invaginates and buds into the cell as a vesicle

Phagocytosis

endocytosis of undissolved / insoluble molecules

1) contact made and surface receptors bind

2) pseudopods reach around and enclose food in membrane → phagosome

3) phagosome + lysosome = phagolysosome → acid digestion

Apical vs Basolateral

Apical faces exterior, cavity, or lumen, often contains microvilli / cilia

Basolateral faces blood, interstitial fluid or adjacent cells

4 main cell to cell connections

• component proteins

• examples

Tight junctions = watertight seal, cell membranes connected

• made of claudins and occludins

• found in bladder (hold pee), intestines, blood-brain barrier

Desmosomes = spot weld, permeable to fluid and small molecules

• made of cadherins, anchored to intermediate filaments

• found in skin, cardiac muscle (bind cells and distribute mechanical stress), intestines

Adherens junctions

• also made of cadherins, but anchored to actin cytoskeleton

• found in epithelial cells, epidermis, blood vessels and cardiac muscle; form ‘belts’ that regulate cell shape and maintain tissue integrity, though not as strong or flexible as desmosomes

Gap junctions = Tunnels, connect cytoplasm of 2 cells, allowing rapid, direct passage of water / ions / messengers

• made of connexins

• found in cardiac and smooth muscle (for coordinated contractions) and neurons (for coordinated firing)

• eg. plasmodesmata in plants

Plasmodesmata

Gap junctions for plants

G-protein coupled receptor

• what is it

• how does it work

Receptor protein with 7 transmembrane alpha helices

• Initially, receptor bound to G-protein’s alpha subunit bound to GDP

• Ligand binds, forming ligand-receptor complex → conformational change → G-protein alpha subunit swaps GDP for GTP and dissociates

• alpha subunit & GTP go phosphorylate protein/enzyme

• Activated enzyme activates secondary messenger, which relays signal to cellular machinery

• When G-protein hydrolyzes GTP back to GDP, it reassociates into inactive trimer and interacts with GPCR to release ligand

common enzyme = Adenylyl cyclase

common secondary messenger = ATP → cAMP

Receptor Tyrosine Kinase

• what is it

• how does it work

• Enzyme-linked receptor that binds signaling proteins (usually growth hormones)

• Ligand binds → 2 RTKs cross-link and dimerize → tyrosine residues get phosphorylated by the other RTK → phosphorylated tyrosines serve as binding sites for signaling proteins (help shi phosphorylate and activate each other i guess)

• can be turned off by dephosphorylation

Give the major roles of these organelles:

Nucleus, Mitochondria, Rough ER, Smooth ER, Golgi apparatus, Lysosome, Peroxisome

Nucleus contains DNA, makes ribosome parts

Mitochondria does cellular respiration, generates heat and gatekeeps apoptosis

Rough ER makes, folds and quality-controls proteins, especially organelle, membrane and secretory proteins (cytosolic ribosomes make cytosolic proteins)

Smooth ER produces lipids, does detox, carbohydrate breakdown and stores calcium

Golgi apparatus is the ‘post office’ that modifies, sorts and ships proteins to specific destinations, also helps make lysosomes

Lysosome breaks down stuff (organelles, proteins, lipids, carbohydrates, pathogens, other waste) with an acid interior (pH 4.5 to 5.0) full of hydrolytic enzymes, help with apoptosis

Peroxisome breaks down long and/or branched chain fatty acids, makes special lipids, detoxifies H₂O₂ ft. catalase

Name the 3 main components of the cytoskeleton and describe their component materials and characteristic functions

Microfilaments

• made of actin

• intracellular transport (ft. myosin), motility (ameboid movement, muscle contraction) cytokinesis (actomyosin ring pinches cells apart)

Intermediate filaments

• material depends (eg. keratin in hair and nails, lamin in nucleus, desmin, neurofilaments, etc.)

• Mostly provide structural support, also help w/ cell migration & subcellular organization

Microtubules

• dimer of α and β tubulin

• intracellular transport ft. dynein and kinesin, form mitotic spindle, cilia and flagella, structural support

Gram-positive vs. Gram-negative bacteria

Thick peptidoglycan cell wall

• Gram stain → stained purple

• Generally more susceptible to antibodies

vs.

Thin peptidoglycan cell wall + outer membrane w/ lipoproteins & polysaccharides

• Gram stain → stained pink

• Generally more resistant to antibodies, outer membrane acts as barrier

Binary fission vs. Mitosis

Binary fission does not involve mitotic spindle, binary fission has DNA duplicate during replication instead of before

4 methods of prokaryotic genetic variation

Transformation: pick up free DNA

Conjugation: bacteria sex (transfer via direct contact)

Transduction: viruses

Transposable elements: hop between chromosome and plasmids

Describe and name the 2 ‘processes’ that prevent our body’s stem cell supplies from being depleted.

Obligate asymmetric division = stem cells divide to produce 1 differentiated cell and 1 ‘replacement’ stem cell

Stochastic differentiation = when a stem cell divides to produce 2 differentiated cells, a neighboring stem cell takes notice and divides to produce 2 ‘replacement’ stem cells